R help - Dec 2011 - sapply Call Returning " the condition has length > 1" Error

Dear all,

Happy new year!

I have a question re using sapply. Below is a dummy example that would replicate
the error I saw. 

##Code Starts here
DummyFunc <- function(x) {

if (x > 0) {
return (x)
} else
{
return (-x)
}

}

Y = data.frame(val = c(-3:7))
sapply(Y, FUN = DummyFunc)
##Code ends here

When I run it, I got:
     val
 [1,]   3
 [2,]   2
 [3,]   1
 [4,]   0
 [5,]  -1
 [6,]  -2
 [7,]  -3
 [8,]  -4
 [9,]  -5
[10,]  -6
[11,]  -7
Warning message:
In if (x > 0) { :
  the condition has length > 1 and only the first element will be used

The result is different from what I would expect plus there is such an error
message.

I guess if the DummyFunc I provided is compatible with vectors, the problem
would go away. But let's suppose I cannot change DummyFunc. Is there still a
way to use sapply or alike without actually writing a loop? Thanks.

- Alex
	[[alternative HTML version deleted]]

You are right that the problem is that "DummyFunc" isn't
vectorized. R
looks for a single logical value in an "if" statement but
"x>0" gives
it a whole vector's worth -- as the warning indicates, it only uses
the first and pushes the whole vector through the loop in the
return(-x) branch, which explains the values you saw. The correct way
to do it would be something like:

ifelse(x < 0, -x, x)

If, as you suggest, you can't modify the function (for whatever
reason), you can use the higher-order-function Vectorize() as follows:

vDummyFunc <- Vectorize(DummyFunc)
vDummyFunc(-3:7)

This isn't real vectorization, but it hides some *apply family stuff nicely.

Note that this doesn't act as you might expect on Y since data.frames
are taken column wise by default (you'll get the same problem).

Michael

On Tue, Dec 27, 2011 at 1:14 PM, Alex Zhang <alex.zhang at ymail.com>
wrote:
> Dear all,
>
> Happy new year!
>
> I have a question re using sapply. Below is a dummy example that would
replicate the error I saw.
>
> ##Code Starts here
> DummyFunc <- function(x) {
>
> if (x > 0) {
> return (x)
> } else
> {
> return (-x)
> }
>
> }
>
> Y = data.frame(val = c(-3:7))
> sapply(Y, FUN = DummyFunc)
> ##Code ends here
>
> When I run it, I got:
> ? ? ?val
> ?[1,] ? 3
> ?[2,] ? 2
> ?[3,] ? 1
> ?[4,] ? 0
> ?[5,] ?-1
> ?[6,] ?-2
> ?[7,] ?-3
> ?[8,] ?-4
> ?[9,] ?-5
> [10,] ?-6
> [11,] ?-7
> Warning message:
> In if (x > 0) { :
> ? the condition has length > 1 and only the first element will be used
>
> The result is different from what I would expect plus there is such an
error message.
>
> I guess if the DummyFunc I provided is compatible with vectors, the problem
would go away. But let's suppose I cannot change DummyFunc. Is there still a
way to use sapply or alike without actually writing a loop? Thanks.
>
> - Alex
> ? ? ? ?[[alternative HTML version deleted]]
>
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

Dear Alex,
> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Alex Zhang
> Sent: December-27-11 2:14 PM
> To: r-help at r-project.org
> Subject: [R] sapply Call Returning " the condition has length >
1"
> Error
> 
> Dear all,
> 
> Happy new year!
> 
> I have a question re using sapply. Below is a dummy example that would
> replicate the error I saw.
> 
> ##Code Starts here
> DummyFunc <- function(x) {
> 
> if (x > 0) {
> return (x)
> } else
> {
> return (-x)
> }
> 
> }
> 
> Y = data.frame(val = c(-3:7))
> sapply(Y, FUN = DummyFunc)
> ##Code ends here
> 
> When I run it, I got:
>      val
>  [1,]   3
>  [2,]   2
>  [3,]   1
>  [4,]   0
>  [5,]  -1
>  [6,]  -2
>  [7,]  -3
>  [8,]  -4
>  [9,]  -5
> [10,]  -6
> [11,]  -7
> Warning message:
> In if (x > 0) { :
>   the condition has length > 1 and only the first element will be used
> 
> The result is different from what I would expect plus there is such an
> error message.
This is a warning, not really an error message. A data frame is essentially
a list of variables (columns), and sapply() applies its FUN argument to each
list element, that is, each variable -- the one variable val in your case.
That produces a warning because val > 0 is a vector of 11 elements, and the
first comparison, 3 > 0, which is TRUE, controls the result.
> 
> I guess if the DummyFunc I provided is compatible with vectors, the
> problem would go away. But let's suppose I cannot change DummyFunc. Is
> there still a way to use sapply or alike without actually writing a
> loop? Thanks.
Well, you could just use
> abs(Y$val)
 [1] 3 2 1 0 1 2 3 4 5 6 7

but I suppose that you didn't really want to write your own version of the
absolute-value function as something more than an exercise.

An alternative is
> with(Y, ifelse(val > 0, val, -val))
 [1] 3 2 1 0 1 2 3 4 5 6 7

I hope this helps,
 John

--------------------------------
John Fox
Senator William McMaster
  Professor of Social Statistics
Department of Sociology
McMaster University
Hamilton, Ontario, Canada
http://socserv.mcmaster.ca/jfox


> 
> - Alex
> 	[[alternative HTML version deleted]]